1. Field of the Invention
The present invention relates to a composite structure that the circumference of a core material made of a sintered diamond is coated with a shell layer made of a sintered alloy.
2. Description of Related Art
Such a technology has been researched that improves hardness, strength and toughness of a structure by coating an elongate core material such as filament with other material. Japanese Unexamined Patent Publication (Kokai) No. 11-139884, for example, discloses a sintered composite ceramic made by coating the circumference of a core material made of ceramic (filament-like ceramic) with a coating layer of a second component by spraying, bundling the coated core materials, subjecting the assembly to compression molding and sintering it, in order to increase the fracture resistance of the structure.
On the other hand, sintered diamond material comprising diamond particles bound by an iron-group metal has been used in cutting tools, mining tools and abrasion resistant parts, taking advantage of the high hardness of diamond. U.S. Pat. No. 6,063,502 discloses a composite structure comprising a core material made of a sintered diamond with a shell layer made of WC—Co being provided on the circumference thereof.
However, the sintered diamond material of the prior art described above has high hardness but low toughness and low impact resistance, and is poor in chipping resisting performance for the applications as cutting tools or mining tools.
In the case of a composite structure comprising a core material made of a sintered diamond coated with a shell layer made of a sintered alloy such as cemented carbide (WC) constituted mainly from metal of the group 4a, 5a or 6a of the Periodic Table such as that described in the U.S. Pat. No. 6,063,502, particularly when the mean particle size of diamond particles in the core material is made small in order to increase the strength, balance between the infiltration of the binding metal and the wettability of the diamond particles is lost, resulting in a region deficient of the binding metal being formed in a considerable area in the core material in the interface with the shell layer. Presence of such a region where the binding metal is distributed unevenly results in a lower strength of the structure. A cutting tool made of such a material has low wear resistance and low adhesion resistance. Moreover, chipping resistance may significantly decrease when the direction of filament orientation in the structure deviates even slightly from the direction of the cutting edge resulting in significant decrease in the binding force between the filaments.